This invention relates to fiber optic communication systems and particularly to fiber optic communication systems for use in industrial control applications. More particularly, the invention relates to an optical communication system which continues uninterrupted communication between various elements of an industrial control system even in the event of a failure in the fiber optic lines.
Industrial control systems are used in industrial processes to control pressures, temperatures, mass transfers, and other process parameters. Control systems may included many input/output devices such as temperature sensors, pressure sensors, pressure regulators, and other similar devices positioned at various locations in the process equipment being controlled. Modern control systems also include one or more programmable logic controllers ("PLCs") for controlling various input/output devices in the system, based upon data collected from the input/output devices and based upon instructions programmed on the controller.
These input/output devices and PLCs must communicate with each other in order for the control system to provide the desired control for the process. Both data and instructions must be communicated between devices in the control system. Thus, an industrial control system requires a communication arrangement to enable the various control system devices to communicate with each other.
Some industrial processes may require very precise control. Failure of communications between the various elements of an industrial control system may upset the entire process and have dire results. For example, failure of process control communications in a chemical manufacturing process may result in the production of an entirely different chemical than what was intended. Thus, a communications failure in an industrial control system may require that the entire process being controlled be aborted or at least suspended until communication is reestablished. Also, failure of communication and control in some processes may pose very serious safety risks. Therefore, the communication systems employed in industrial control systems must be very robust.
Yet the communication lines required between the various elements of an industrial control system must commonly traverse harsh environments or areas of high activity. In these areas, there is a constant danger of damage to the communication lines resulting in a loss of communication in the control system. Also, conditions in some areas of an industrial process may interfere with certain types of transmissions. For example, electrical noise in certain areas of a plant may interfere with electrical transmissions. In these situations and others, data and instructions may be converted to optical signals for transmission between the various control system devices through optical fibers.
The various elements of an industrial control system are advantageously connected for communications in a ring configuration allowing communications in both directions around the ring. In this closed ring configuration, a single break in the ring does not cause a loss of communications since communication is still possible in the opposite direction from the location of the break. Closed communications rings, however, raise the problem of signal oscillation which interferes with communications.
One way to solve the problem of ensuring communications in the event of a failure along a communication path in an industrial process control system is to provide multiple communication paths between the various elements of the system. In the event of a failure on one communication path, communications are still maintained along a secondary or tertiary path. However, systems having multiple communication paths require duplicate communication lines and associated material and are therefore more costly. Furthermore, multiple communication paths complicate communication protocols.
Detecting a failure of communication in an industrial control system process is complicated by the nature of communications in the system. While some communications of data and instructions may be relatively continuous in some process control applications, communications may be infrequent in other control systems. In situations in which long gaps appear between data or instructions, a communication failure may not be readily apparent. A monitoring system which monitors for communication failures may mistake a communication fault for a gap in data, or mistake a gap in data for a communication fault.